It's easy! First, just unscrew the weapon and pull the halves apart. You'll want to trim off the tabs on the very bottom that hold the clear orange piece in place:

You can also trim some material away from the base of the screw posts to get a little extra size from the flames. Be careful not to cut TOO deep into the screw cavity itself, but you don't have to worry too much about being pretty, since it's all interior.

Next, take the flat part of the orange bit:

You'll want to trim a bit off of the side tabs and remove a lot from the top to allow it to slide up. You'll also want to angle it more on the sides, so it doesn't get stuck on the inner curvature of the mace. When you're done it should look like this:

Now go to the flame side:

What you want to do here is cut down to the thicker bits at the bottom of the piece so there are grooves for the screw posts to slide in. It should look something like this when you've got it done:

Remember to test fit throughout the entire process as you're cutting. It makes a WORLD of difference when you're done, and helps prevent mistakes. It's the equivalent of saving in an RPG. DON'T LOSE ALL YOUR PROGRESS BECAUSE YOU DIDN'T KEEP AN EYE ON YOUR WORK!